The present invention relates to a method of determining the residual life of mechanical structures, especially those of which the fatigue strength is in question, making use of X-ray.
Generally, the structural members constituting a mechanical structure are designed to be subjected to stresses of levels below the fatigue limit of the material. In the actual use, however, the structural member is often subjected to stresses of levels higher than the designed stress for various reasons, and is failed due to such stresses. In order to prevent such a failure, it is necessary to determine the residual life of the structural member being used.
Various methods have been proposed hitherto for determining the residual life of the structural member. For instance, according to one known method a metal foil is adhered to the structural member of which the fatigue strength is in question, or a test piece having a plurality of notches with more than two different stress concentration coefficients is attached to the structural member. Then, after the fatigue, the X-ray diffraction intensity is measured with the metal foil or the test piece. The residual life of the material or the extent of damage is then detected through the calculation of the integrated value of the X-ray diffraction intensity profile or from the amount of change in the halfvalue width. This conventional method, however, necessitates the attaching of the metal foil or the test piece to the structural member to be examined.
Further, in most cases it is not possible to attach the metal foil or test piece to the structural member under the use. Even when the attachment is possible, the method relying upon the adhesion of the metal foil cannot detect the damage due to fatigue caused by compression stress.
Under this circumstance, the present applicant has proposed in Japanese Patent Application Laid-open Publication No. 20894/76 a new method comprising the steps of determining the relationship between the stress amplitude and the stress number till rupture, from the rate of change in residual stress obtained through an X-ray diffraction, detecting the ratio of the amount of change in residual stress to the initial residual stress through measurement of position by the X-ray diffraction profile, and calculating the number of repetition of stresses till failure. This method, however, necessitates the determination of ratio of amount of change in the residual stress to the initial residual stress, which in turn requires a stress measurement by X-ray. It is quite troublesome and difficult to conduct this measurement.
In another known method for determining the residual life, a strain guage is adhered to the surface of the structural member of which the fatigue strength is in question to measure the stress in the structural member. The measured stress values are compared with the values in a S-N curve (a curve of stress vs. number of repetitions of stress) which is previously drawn through experimental fatigue tests carried out in a laboratory, and the limit of use, i.e. the residual life, is determined by reading the number of repetition of stress from the S-N curved. As a matter of fact, however, the structural member often makes a motion such as rotation. In such a case, it is not possible to adhere the strain gauge to the structural member to be examined.
The X-ray diffraction method, therefore, has been proposed as a method which makes it possible to measure the degree of damage due to fatigue without direct measurement of the stress.
Briefly, this method is to make use of such a phenomenon that the halfvalue width of an X-ray diffraction intensity profile obtained by irradiating an X-ray to the object varies depending on the distortion attributable to fatigue. Namely, the degree of damage is detected as the ratio of the halfvalue width before the fatigue to the halfvalue with after the fatigue.
Hitherto, when a damage is found out in a structural member, the latter is replaced with a new member without delay, because the damage is expected to develop to cause a failure sooner or later. It takes a considerably long period of time for the renewal of the structural member. Also, the fabrication of new structural member incurs a rise of cost.
In contrast, it is a current measure to determine the residual life of the structural member, from the detection of degree of the damage, and to use the member almost to the expiration of the life thereby to reduce the cost incurred. The determination of the residual life can be made by a conventional method using X-ray but this known method involves various problems. More specifically, this method is to determine the stress applied to the member from the halfvalue width ratio and, using the stress thus obtained, the number of repetition of stress till rupture from an S-N curve. Then, the residual life is calculated by subtracting the number of repetition of stress till the measurement of the halfvalue width from the number of repetition of stress till failure. Alternatively, by making use of a fact that the halfvalue width ratio corresponds in certain extent to the number of repetition of the stress, the residual life is determined from the halfvalue width ratio and the number of repetition of stress. As a matter of fact, however, there are only few cases that the record of change in stress in a structural member is available. Therefore, it has been almost impossible to know the residual life by the known methods.